1. Field of the invention
This invention pertains generally to pumps and motors, and more particularly to a positive displacement cavity pump or motor having a stator with interchangeable rotors to vary cavity displacement.
2. Description of the Background Art
Hydraulic motors and pumps are respectively used in oil fields for drilling oil wells and pumping the production fluid from the wells. One commonly used design of pump and motor is a Progressing Cavity design as taught in U.S. Pat. No. 1,892,217 and incorporated herein by reference. The Progressing Cavity design employs a rotor that rotates and nutates within a stator and is widely accepted in the oilfield industry because of its ability to produce high torque and low speeds as a motor, and its ability to withstand abrasion and develop high pressures at low shear rates as a pump. The rotational and nutational directions oppose each other, i.e. if the rotor rotates clockwise around its axis, the nutational motion around the center of the stator is counter-clockwise.
The geometry of the rotor and stator are most commonly modified hypocycloids, although other geometric shapes can also be used. The rotor consists of "n" lobes and the stator has "(n+1)" lobes, however, the minimum number of lobes in the stator is two. For example, the rotor can have four lobes while the stator has five lobes. The lobes are helical longitudinally, and the distance between two successive peaks (or valleys) is the pitch. The lead is the longitudinal distance between the same location, such as a peak, on one lobe for 360.degree. rotation of that lobe. The lead is equivalent to the pitch multiplied by the number of lobes. In U.S. Pat. No. 1,892,217, the pitch of the rotor and the stator are the same, however, the leads are in the ratio of the number of lobes. Therefore, the lead of the stator is always greater than that of the rotor. The rotor engages in the stator offset from the center of the stator by an amount known as "eccentricity". This engagement results in multiple pockets (or chambers) for one lead of stator, and fluids are transported through these pockets. In the case of motors, fluids in these pockets act on the rotor to create a torque forcing the rotor to rotate and nutate, provided the torque generated is greater than the load's resistive torque. In the case of pumps, rotation of the rotor causes the fluid in the pocket of one lead to migrate to the pocket in the next successive lead of the stator. The direction of travel of fluid depends on the "hand" of the helix and the rotation of the rotor.
Such a design dictates that a stator with given design parameters, such as major diameter, minor diameter and lead, mesh with only one rotor. In a commonly known configuration where the stator has two lobes and the rotor has one lobe, the lead of the rotor is one-half of that of the stator, the minor diameter of the rotor is the same as that of the stator (ignoring compression), and the eccentricity of the rotor is the same as that of the stator (ignoring compression). As such, for a particular design of rotor and stator, the volume displaced by one rotation of rotor remains fixed.
A problem encountered with such "fixed" designs is the inability to change volume displacements to accommodate varying needs. As an example in pump applications, when a well is new, there is more flow into the well, and it would be more beneficial to pump as much fluid as possible for a given rotational velocity of the rotor. Over time, as the well depletes, flow to the well decreases, and the amount of fluid pumped would have to be reduced to avoid running the pump dry. Therefore, it would be advantageous to be able to change only the rotor to meet the existing well requirements without having to replace the entire pump assembly.
In applications where heavy oil is being pumped, the pump has to run at a very low rotational velocity. In such situations, it is beneficial to pump as much volume as possible from the well. In order to meet both requirements, a larger pump must be used, as long as the well casing is large enough to accommodate a larger pump. Otherwise, maximum output cannot be realized. For such situations, an interchangeable rotor that would function with the given stator, is required.
If the stator and rotor were functioning as a motor for drilling, the rotor must be capable of operating at varying rotational speeds, depending on the particular application. For high speed operation, the one-lobed rotor is used in conjunction with a two-lobed stator (1:2). For low speed operation, a "multilobe" design is used wherein the rotor has between two and nine lobes, and the stator has one more lobe than the rotor. The "multilobe" design reduces the rotational speed of the rotor as compared with the 1:2 configuration, given the same fluid volume input used to drive the rotor.
Therefore, there exists a need for a compact positive displacement pump and motor which has a three-lobed rotor capable of being used interchangeably with a one-lobed rotor within a two-lobe stator, for varying rotational speed requirements and /or fluid volume outputs or requirements. The present invention satisfies those needs, as well as others, and overcomes the deficiencies in prior technology.